As known, natural gas is a mixture of gaseous hydrocarbons, primarily of methane, ethane, propane, and butane, which also contains minor amounts of heavier hydrocarbon components along with varying amounts of gaseous non-hydrocarbon components, such as nitrogen, carbon dioxide and hydrogen sulfide.
Natural gas emerged from several gas fields is "acidic", i.e. it contains considerable amounts of carbon dioxide and hydrogen sulfide. Of them carbon dioxide is a corrosive component which also impairs the caloric value of natural gas, whereas hydrogen sulfide, along with the other sulfides optionally present, produces corrosive and poisonous substances upon combustion. Removal of acidic components is the most frequent goal of natural gas purification, which is performed by using physical or chemical absorbents optionally in combination with one another.
Upon physical absorption carbon dioxide and hydrogen sulfide dissolve practically only physically in the solvent applied as absorbent. When absorption is over, pressure is decreased considerably whereupon gaseous components get desorbed in their original state. The solvent is then recycled. Organic solvents of high boiling points, such as polyethylene glycol dimethyl ether (Selexol) or tetrahydrothiophene-1,1-dioxide (Sulfolan) are used as physical adsorbents.
In chemical absorption methods aqueous solutions of various alkanolamine compounds, such as of monoethanol amine (MEA), diethanol amine (DEA). diisopropano amine (DIPA), diglycol amine (HOCH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 NH.sub.2, DGA) and methyl diethanol amine [(HOCH.sub.2 CH.sub.2).sub.2 NCH.sub.3, MDEA] are utilized, which chemically, bind the acidic components to be removed in the form of adducts (Ullmann's Encyclopedia Vol. A12, pp. 258-259). Solvent regeneration is based on the phenomenon that an increase in temperature and a decrease in pressure runs with the decomposition of the complex, whereupon the acidic gas liberates.
The advantages of chemical absorption (recovery of COS, low level of methane co-absorption) can be increased with the simultaneous use of physical absorbents.
Of the monoamines utilized as chemical absorbents primary amines (monoethanol amine and diglycol amine) are the most reactive ones, therefore these substances (particularly monoethanol amine which is much more easily available than the other one) have been utilized for several years almost exclusively to remove hydrogen sulfide and carbon dioxide from natural gas and from certain synthesis gases. Although these substances yield gases of high final purity, it is disadvantageous that they produce corrosive products with carbon oxysulfide and carbon disulfide, and corrosion also appears after the uptake of a certain amount of carbon dioxide. In order to eliminate this problem various corrosion inhibitors are added to the absorbent solution. Examples of such methods are the UCAR process and the "Amine Guard" process [1989 Gas Process Handbook (Hydrocarbon Process, Gulf Publishing Co., Houston, 1989)], both developed by Union Carbide, of which the second one can be applied to remove carbon dioxide only, because the sulfur-containing components react with the inhibitors utilized. However, the second serious disadvantage connected to the use of primary amine solutions, i.e. that they recovery runs with a great energy consumption due to the high absorption-resorption reaction heat, cannot be eliminated by these methods.
Due to the disadvantages outlined above primary monoamines have been gradually replaced by secondary and tertiary monoamines. Of theni secondary monoamines. such as diethanol amine and diisopropanol amine, are less reactive than the primary amines, but their reactions do not lead to corrosion, and the energy demand of the recovery of secondary amine solutions is less by 20-30% than that of primary amine solutions. However, the usability of secondary amines is more restricted than that of primary amines, because secondary amines react with carbon dioxide (which is the most frequent component to be removed) less quickly than with hydrogen sulfide. Methyl diethanol amine (a tertiary amine) has more advantageous selectivity characteristics than secondary amines, its corrosivity is very low (in carbon steel equipment even a load of 0.8 mole of acidic gas/1 mole of MDEA is admissible), and the recovery of its aqueous solution is less energy-intensive than that of secondary amine solutions. Therefore recently this compound is utilized most frequently to purify acidic gases by chemical run with considerable losses in amine. As a further disadvantage, their recovery is rather energy-intensive, and energy can be saved only on the account of a decrease in reactivity.
It appears from the test data reported in international patent application published under No. WO 89/11327 that certain amino compounds, including polyamines of the formula EQU H.sub.2 N--(/CH.sub.2 /.sub.2-6 --NH).sub.1-6 --(CH.sub.2).sub.2-6 --NH.sub.2 (A),
can be used as activators to accelerate the reaction rate of tertiary alkanolamines. However, the energy demand of recovery remains still relatively high, and recovery of the ternary alkanolamine/activator/water mixture is rather complicated. DE 2815447 reports that some representatives of the above compounds have excellent CO.sub.2 binding ability. No information can be derived, however, from this reference on the reaction rate and on the energy demand of recovery.
There is still a need for improving the economy of purification of gases containing acidic components. More particularly, an absorbent composition is required which has a high reaction rate and can be recovered by an easy and non energy-intensive way with minimum losses.
As a result of our investigations we have found that aqueous solutions of polyalkylene polyamines of the general formula ##STR2##
wherein
n is 2 or 3, PA1 the R.sup.1 groups represent methyl or one of them is hydrogen and the others are methyl, and PA1 R.sup.2 is hydrogen, methyl or a group of the formula --(CH.sub.2).sub.2-3 --N(CH.sub.3).sub.2, can be utilized very advantageously for the purification of gases containing acidic components, primarily for rendering natural gas free of carbon dioxide. PA1 they bind carbon dioxide very quickly, PA1 under low pressures their carbon dioxide binding ability is much greater than that of methyl-diethanol amine, PA1 polyalkylene polyamines of formula (I) are less volatile, thus only minor losses in amine occur at the recovery of the scrubbing solution, which is advantageous both economically and from the aspects of environmental protection, PA1 the toxicity of the compounds of formula (I) is very low, which involves a further advantage from the aspects of environmental protection, PA1 the energy demand of recovery is low.
We have not found any reference to this use of polyalkylene polyamines of formula (I) in the literature. Soviet Patent No. 1 378 901 states that when natural gas is desulfurized with sodium hydroxide, the efficiency of purification can be increased and the corrosion of the equipment can be suppressed if the aqueous sodium hydroxide solution is admixed in a volume ratio of 1:(1-3) with a waste liquor obtained in alkaline desulfurization of liquefied gases, and a polyalkylene polyamine is added to the mixture in an amount of 0.5-2.5% by volume. Thus this paper relates essentially to the reuse of waste liquors. Compounds of formula (I) have been utilized in this process as corrosion inhibitors without even mentioning their possible absorbent effect.